1. Field of the Invention
The present invention relates to a low noise amplification circuit which amplifies high-frequency and weak signals received by means of antennas, and especially relates to a low noise amplification circuit which comprises at least one filter coupled to the input terminal of a low noise amplifier to remove undesired frequency components.
2. Description of the Related Art
Generally, wireless instruments, such as cellular phones or GPS devices, include at least one receiving device with a low noise amplifier (LNA) which amplifies signals within predetermined frequency bands received by means of antennas. However, the antennas receive not only signals within a desired frequency band, but also signals with undesired frequency components near the desired frequency band. For instance, when another wireless instrument is placed close to the receiving device, and transmits or receives radio waves with so large electric field, the undesired frequency components might be received with so large level compared to the signals within the desired frequency bands. As a result, it is likely that the LNA is saturated with the undesired frequency components such that the LNA cannot amplify the signals within the desired frequency bands.
In order to prevent the problem described above, there is proposed a receiving device in which undesired components are removed by at least one filter coupled to the input terminal of the LNA (ex. JP07-058657A, JP2002-164803A). JP07-058657A discloses a receiving device in which undesired components are removed by a band-pass filter coupled to a front end of the LNA. JP2002-164803A discloses a receiving device in which specific frequency bands close to desired frequency bands are removed by a trap circuit arranged between the antenna and the LNA as a band-stop filter.
In the conventional receiving device as described above, the filter coupled to the input terminal of the LNA yields an insertion loss for the signals within the desired frequency band. In general, since a noise figure of a whole circuit is roughly determined due to a noise figure of the first stage of the circuit, the loss yielded before the LNA significantly affects the noise figure of a whole circuit. Accordingly, when a filter is arranged at the front end of the LNA , the insertion loss of the filter directly leads to the deterioration of the noise figure of the whole circuit. When the noise figure is deteriorated due to the filter arrangement, a receiving quality is deteriorated in a receiving instrument for, in particular, the digital modulation system. More specifically, the FIG. 13 shows that the transmitting quality or the transmitting speed is degraded because a bit error rate is significantly deteriorated along with a signal-noise ratio in various digital transmitting systems. When the circuit is applied to a receiving instrument for satellite signals, the communication area might scale down due to the degradation of minimum receiver sensitivity. To solve the problem, it is possible to employ a high performance LNA with excellent noise characteristic, however, it is inevitable that the cost must run much high. When a high performance filter is employed in order to reduce the insertion loss of the filter, a similar problem occurs.